This invention relates generally to the asexual or clonal propagation of superior strains of forest trees. More particularly, it relates to a process for the in situ activation of the normally dormant needle fascicles of gymnosperms and for the clonal propagation of superior strains of gymnosperms, especially the various conifers, including species of the genus pinus, and the clones produced thereby.
For many years land managers within the forest products industry have been reforesting cut-over land in order to produce new crops of timber or pulp wood trees. Initially, reforesting was done naturally--by leaving individual mature trees or blocks of trees on the cut-over land to provide a natural seed source for future generations. It is plain that this technique of natural reseeding left much to chance--e.g., the production of acceptable seed crops by many species of gymnosperms occurs only infrequently.
A preferred method of reforesting is the planting of seedlings grown in nurseries. However, this method is beset by several disadvantages. For example, if wild seeds are relied on for the production of seedlings, they must be collected in the years of good seed crops. In addition, since the wild seeds originate from a large and varied genetic pool, the seedlings are likely to be of uneven quality, particularly in the important commercial characteristics of growth rate and wood or pulp production. To avoid this problem, selected trees, having desired growth rates and product characteristics, were chosen as seed sources for nursery-grown seedlings. However, genetic improvement of tree species using this practice is very slow--it may take 15-50 years for a new generation of trees to produce seeds of its own.
Since the object of any forest tree breeding program is to be able to supply genetically superior seedlings by the hundreds of millions in the shortest time possible, it is critical to reduce the generation time of stock in the breeding program.
One method to reduce the generation time of tree stock is described in U.S. Pat. No. 4,199,897. There, juvenile phase forest species within the genus pinus are treated by varying the growth conditions so as to induce flowering and hence seed production earlier in the species growth cycle. Such process is stated to permit collection of seeds as much as 10-15 years earlier in the growth cycle.
A second method to reduce the generation time of tree stock is the clonal propagation of the stock. In this general method, tree stock is reproduced asexually so that the time of flowering and seed production are less important. Several techniques for the clonal propagation of various species of gymnosperms exist. These include grafting, tissue culture and the rooting of cuttings. Grafting is labor intensive and is disadvantaged by graft (stock-scion) incompatibility problems. It is, therefore, commercially difficult. Tissue culture techniques, such as those described in U.S. Pat. No. 4,217,730, are beset by a number of problems including growth difficulties and the expense and trouble associated with the various culture steps. The rooting of cuttings, including needle fascicles (dwarf shoots), as described in e.g. [R. J. Hoff and G. I. McDonald, USDA Forest Service Research Note 80 (1968); F. E. Larsen and R. W. Dingle, Forest Sci., 15, pp. 64-65 (1969)], is disadvantaged because the percentage of success is usually very low--the cutting or bud is more often than not unable to grow into a shoot. Furthermore, only one plantlet/branch can be obtained by rooting cuttings.
Several attempts have been made in the prior art to improve the percentage of success in the rooting of needle fascicles. For example, the pruning of terminal buds from long shoots has been shown to stimulate shoot development of needle fascicles [R. J. Hoff and G. I. MacDonald, supra; F. E. Larsen and R. W. Dingle, supra; F. Mergen and B. A. Simpson, Silvae Genet., 13, pp, 133-39 (1964); R. C. Hare, J. Forestry, 63, pp. 544-46 (1965)]. Needle fascicles have also been activated to form shoots by foliar sprays of kinetin in Pinus radiata [J. Kummerov and C. A. Hoffman, Ber. Dtsch. Bot. Ges., 76, pp. 189-96 (1963)] and 6-benzylamino-purine ("BAP") in Pinus elliotti [M. M. Concha and E. R. Montaldi, Idia Supl. For. 3, pp. 49-53 (1966)], Pinus sylvestris [S. J. Whitehill and W. W. Schwabe, Physiol. Plant., 35, pp. 66-71 (1975)], Pinus ponderosa and Pinus strobus [M. A. Cohen and J. Shanks, J. Amer. Soc. Hort. Sci., 100, pp. 404-06 (1975); M. A. Cohen, J. Amer. Soc. Hort. Sci., 103, pp. 483-84 (1978)]. However, in each of these cases large volumes of cytokinin solutions were required to be used in the foliar sprays and a limited number of needle fascicle shoots actually were induced. For example, a foliar spray consisting of 50 ml (or more) of a 500 to 1000 mg/l BAP solution (25 to 50 mg BAP) was used every four to five days for a period of 30 days by M. M. Cohen and J. Shanks, supra, and S. J. Whitehill and W. W. Schwabe, supra. In addition, the time period required after treatment with such foliar sprays for shoot elongation before rooting ranged from 6 to 12 months and the rooting frequency of these shoots was low. Therefore, the clonal propagation of tree stock by the rooting of needle fascicles is still not commercially attractive.